The present invention relates to a torque wrench having a rotating angle measurer, and more particularly, to a torque wrench provided with a device to measure screw rotating angles so as to strictly control fastening torque.
There are three known methods as specified in JAPAN INDUSTRY STANDARD B 1083 for strictly controlling fastening torque when threaded parts such as bolts are fastened. According to one of these methods, fastening is carried out with a torque wrench until the fastening torque reaches a specific torque value while the torque wrench is measuring torque. If the specified torque value is reached, the torque wrench will finish fastening.
The second method is called a rotating angle method, in which fastening of the bolt is carried out with a torque wrench to a specified torque value. To complete fastening, the torque wrench is further rotated to a specific angle.
The third method is called a torque gradient method. In this method, the rotating angle of the bolt and the fastening torque are measured at the same time. The fastening of the bolt is carried out to a specific point obtained from the gradient relation between the above described rotating angle and the fastening torque. This shows that the rotating angle of the bolt and the fastening torque to certain values increase with keeping a proportional relation between the fastening angle and the fastening torque. When a rotating angle is, however, increased from a point approximate to an elastic limit either of the bolt or a object to be fastened with the bolt, torque will not be increased proportionally. If the torque wrench is, nonetheless, further rotated, threads of the bolt will be damaged. The fastening torque is indicated on the ordinate in a graph, and the rotating angle is indicated on the abscissa in the graph.
The torque wrench continues to detect the gradient in the graph. When the gradient in the graph approaches the elastic limit, the fastening of the torque wrench is finished. This torque gradient method enables, consequently, an optimal fastening torque to be applied to the bolt, so as to prevent it from being damaged due to excessively tight fastening.
The torque wrench used in this torque gradient method should be such that the rotating angle of a threaded screw part toward the object body and fastening torque can be simultaneously measured.
FIG. 1 shows a block diagram of a conventional torque wrench. The torque wrench 1 as shown therein gives a specific torque to a screw part, such as a bolt 2, to turn it to secure object bodies 3a and 3b.
The torque wrench 1 comprises of an operating lever 4a, a metallic molded wrench body 4 having a rotary part 4b (rotatable 360 degrees) provided at the front end of the operating lever 4a, a ratchet 5 consisting of a ratchet wheel 5a and a ratchet pawl 5b provided on the rotary part 4b, as a one-way rotation restrict mechanism, an output shaft 6 projecting downward from the center of the ratchet wheel 5a, and a socket 7 fitted onto an output shaft 6 for receiving the head of the bolt 2.
The ratchet wheel 5a is provided so as to be rotatable in two directions at the rotary part 4b. Thus, the ratchet wheel 5a is engaged with the ratchet pawl 5b provided at the rotary part 4b, whereby the ratchet wheel 5a rotates only in a single direction with respect to the rotary part 4b.
A bridge circuit (not shown) comprises strain gauges 8 and 9 provided on both sides. The strain gauges 8 and 9 are used to measure strain generated at the position where the operating lever 4a is mounted onto the rotary part 4b and measure fastening torque applied to the bolt 2.
A rotary encoder 10 is aligned with the center of the output shaft 6 when the encoder is mounted onto the rotary part 4a. A rotary part 10a of the rotary encoder 10 is fixed at the rotary part 4b. The rotary part 4b is rotatably provided with a fixture 10b in the rotary encoder 10. The fixture 10b is secured to a object body 3c with a fixing wire 10c and a magnet 10d. The body 3c is not displaced relative to object bodies 3a, 3b.
Using the ratchet 5 which mechanically restricts rotational movements in one direction, rotational movements of the one direction of the operating lever 4a are transmitted to the output shaft 6 to rotate the bolt 2. Any rotational movement in a reverse direction is idled and is not transmitted to the output shaft 6. This will prevent the bolt 2 from rotating in the reverse direction.
Accordingly, the output shaft 6 is rotated in one direction to fasten the bolt 2 by the reciprocal rotational movements of the lever 4a. The rotary encoder 10 detects, the reciprocal rotational movements of the rotary part 4b toward the object bodies 3a, 3b, and 3c. An operator knows the rotating angle of the bolt by means of the rotary encoder 10 from the detected reciprocal rotary movements of the rotary part 4b. The bolt 2 is thus fastened according to the foregoing torque gradient method.
The rotary encoder 10 detects, due to its construction, however, rotational angles of the operating lever 4 in both directions. This means that the rotating angle as measured by the rotary encoder 10 is almost double the rotating angle of the bolt 2. It is necessary for the operator to determine actual rotating angle by deducting the idle rotational angle from the measured rotating angle of the rotary encoder 10. Since such calculations are so complicated, miscalculations may be made at work sites.
In addition, there is a gap for play where the output shaft 6 comes in contact with the socket 7 to facilitate the replacement of such sockets. Movements of the operating lever include a displacement corresponding to such an idle during reciprocating movements. The conventional type torque wrench is, however, unable to detect such displacement and thus the measurements of the detected angles may suffer a lot of errors toward the actual rotating angles.